Current scientific concerns regarding the impacts of global change include the responses
of forest composition and biomass to rapid changes in climate, and forest gap models have
often been used to address this issue. These models reflect the concept that forest
composition and biomass in the absence of large-scale disturbance are explained by
competition among species for light and other resources in canopy gaps formed when
dominant trees die. Since their initiation 25 years ago, a wide variety of gap models have
been developed that are applicable to different forest ecosystems all over the world. Few
gap models, however, have proven to be equally valid over a wide range of environmental
conditions, a problem on which our work is focused. We previously developed a gap model
which is capable of simulating forest composition and biomass in temperate forests of
Europe and eastern North America based on a single model structure. In the present study,
we extend the model to simulate individual tree species response to strong moisture
seasonality and low temperature seasonality, and modify the wide-spread parabolic
temperature response function to mimic non-linear increases in growth with increased
temperature up to species-specific optimal values. The resulting gap model, FORCLIM V2.9,
generates realistic projections of tree species composition and biomass across a complex
gradient of temperature and moisture in the Pacific Northwest of the United States. The
model is evaluated against measured basal area and stand structure data at three
elevations of the H.J. Andrews LTER site, yielding satisfactory results. The very same
model also provides improved estimates of species composition and stand biomass in eastern
North America and central Europe, where it originated. This suggests that the model
modifications we introduced indeed are generic. Temperate forests other than those we
studied here are characterized by climates that are quite similar to the ones in the three
study regions. Therefore we are confident that it is possible to explain forest
composition and biomass of all major temperate forests by means of a single hypothesis as
embodied in a forest gap model.